Method for improving the frictional properties of carbon brushes by low pressure polyethylene impregnation



Aug. 14, 1962 J. MlLLET 3,049,448

METHOD FOR IMPROVING THE FRICTIONAL PROPERTIES OF CARBON BRUSHES BY LOW PRESSURE POLYETHYLENE IMPREGNATION Filed March 10, 1959 ttorneys iIQ I The present invention relates to methods for improving the frictional properties of carbon members, and to applications of these methods.

In the past there have been used methods for improving the properties of brushes operating at high altitudes, which generally comprise the addition to the carbonaceous substance which constitutes the brush of lubricating or hygroscopic substances. The purpose of these substances has been to improve the contact, to lubricate the surface of the collector and thus to avoid rapid wear of the brush.

The assignees of the present application have already protected several processes for lubricating carbon or graphite operating in atmospheric conditions which are dry and rarefied.

I have established that surfaces covered by hydrogen atoms have a particularly small coeflicient of friction. I have obtained this condition by orientation of very long chain molecules retained by absorption on the friction surfaces so as to cover the latter with hydrogen atoms.

The process according to the invention consists in impregnating absorbent carbons with polyethylenes manufactured at low pressure.

Polyethylenes manufactured at high pressure do not melt freely but exhibit a kind of softening under the influence of heat. This drawback due to a proportion of about 60% of subdivided chains renders the impregnation of the carbons difficult on the one hand because it is necessary to put them in solution, and on the other hand does not give to the impregnated carbons a satisfactory coefiicient of friction.

On the other hand polyethylenes manufactured at low pressure have a proportion of less than 20% of subdivided chains Which gives them physical properties which are very different from those referred to in the preceding paragraph. In effect they show a crystalline state favouring friction, a high melting point which is above 110 C., stability to heat and to chemical agents, all of which makes the impregnating material very suitable.

The impregnation is moreover facilitated by the Weak viscosity of low pressure polyethylenes which actually melt above a temperature of above 110 C. or so. This is a property which is well known to those skilled in the art to which this invention relates and is in contradistinction to high pressure polyethylenes which only soften at elevated temperatures and do not melt.

Impregnation is effected by simple immersion in a bath of polyethylene maintained above 110 C. The impregnation may be slow if there is no application of vacuum or pressure. It may, however, be made rapid and also more homogeneous if alternations of vacuum and pressure are utilised.

The introduction of a proportion of l to 8% by Weight of polyethylene manufactured at low pressure into the pores of a brush made from graphite for example, considerably improves its frictional properties under normal atmospheric conditions, but even more so at low pressures. Thus a brush made from non-impregnated graphite has a coeflicient of friction of 0.50 and a wear equal to 2 mm. per hour. After impregnation these characteristics are as follows: coefficient of friction is 0.20, wear microns per hour during the whole period of the test capable of being carried out for more than 500 hours.

tates Patent The resistance of polythylenes to chemical agents is remarkable. It is thus possible to envisage the utilisation of these impregnation agents for other carbons than those intended for use as brushes in rarefied and dry atmospheres. In elfect carbons impregnated with polyethylenes in accordance with the invention constitute friction members which are completely adapted to use in corrosive surroundings such as joints for the chemical art for example.

In order that the invention may be more clearly understood, reference Will now be made to the accompanying drawings, which show one specific embodiment thereof by way of example together with a cross-section of a brush made in accordance with the method of the invention, and in which:

FIGURE 1 shows a cross-section through an immersion bath adapted for operation in the open atmosphere,

FIGURE 2 shows a section through the bath of FIG- URE 1 but enclosed within a chamber which may be evacuated or placed under positive pressure,

FIGURE 3 shows a cross-section through part of a brush treated in accordance with the invention, and

FIGURE 4 shows the manner in which hydrogen atoms are distributed at the friction surface of a brush portion.

In FIGURE 1 there is shown an absorbent carbon brush 1 for electrical machinery which is placed in a bath 2 containing molten polythenes 3 that have been manufactured at low pressure. This bath is arranged to be heated in any convenient manner as indicated by the arrows in the legend.

FIGURE 2 shows the same arrangement as FIGURE 1 but in this case the bath 2 is enclosed within a chamber 4 which is sealed against the atmosphere and has an outlet 5 whereby said chamber may be connected at will to a pressure pump schematically illustrated at 6 or a vacuum pump schematically illustrated at 7 under control of a valve 8 rotatable within a casing 9 and having a right angle bore 10 as shown. By suitable rotation of the valve body 8 in any desired fashion, e.g. by a spindle passing through the casing 9 to the outside, the bore 10 may be connected as desired either to the pump 6 or the pump 7 by way of the conduit 11 or 112 respectively, and also to the outlet 5 at the same time. In this way the chamber 4 can be evacuated or subjected to positive pressure as desired and the valve 8 may be operated so as to effect these physical conditions alternately: if desired the aforementioned spindle may be connected to a rotation-imparting device of any desired kind so that this alteration may be effected automatically. The speed of rotation could be adjusted to such requirements. The pressure pump 6, the vacuum pump 7 and the valve 8 may be of any kind well known in industry and therefore need not be described here. Similarly the rotation-imparting device may take any desired form, for example may be an electric motor having a controllable speed for driving the aforementioned spindle by means of a belt or gear drive also well known in industry.

The impregnation is effected by means of the apparatus of FIGURE 1 or 2 as referred to above and the degree of impregnation will naturally depend upon the time of immersion and of the amount of pressure or vacuum treatment.

FIGURE 3 shows a portion of a brush made according to the invention in which 13 represents carbon or graphite particles and 14 represents the polyethylene impregnation, exhibiting a friction surface 15.

FIGURE 4 shows the distribution of hydrogen atoms of the polyethylene impregnant which is illustrated by its normal graphic formula. The friction surface is at 15, and 16 represents the metal, generally copper, of the commutator or other machine part with which the brush is to come into contact. It will be understood that the arise 14s brush portion shown in FIGURE 4 is sub-microscopically small and is on a very much larger scale than that of FIGURE 3.

I claim:

1. In a method for improving the friction properties of absorbent carbon brushes for electrical machines by impregnating a brush with a polymeric substance, the improvement which consists in using low pressure polyethylene as the impregnating polymeric substance.

2. In a method for improving the friction properties of absorbent carbon brushes for electrical machines by impregnating a brush With a polymeric substance, the improvement which consists in melting into a brush a low pressure polyethylene, said polyethylene constituting said plymeric impregnating substance.

3. In a method for improving the friction properties of absorbent carbon brushes for electrical machines by impregnating a brush with a polymeric substance, the improvement which consists in immersing a brush in a bath of molten low pressure polyethylene, thereby producing hydrogen atoms on the surface of said brush, said polyethylene constituting said polymeric impregnating substance.

4. A method as claimed in claim 3, in which said brush is subjected to positive pressure during said impregnation.

5. A method as claimed in claim 3, in which said brush is subjected to a vacuum during said impregnation.

6. A method as claimed in claim 5, in which said brush is additionally alternately subjected to positive pressure during said impregnation.

References Cited in the file of this patent UNITED STATES PATENTS 2,384,848 Peters Sept. 18, 1945 FOREIGN PATENTS 721,211 Great Britain Jan. 5, 1955 

1. IN A METHOD FOR IMPROVING THE FRICTION PROPERTIES OF ABSORBENT CARBON BRUSHES FOR ELECTRICAL MACHINES BY IMPREGNATING A BRUSH WITH A POLYMERIC SUBSTANCE, THE IMPROVEMENT WHICH CONSISTS IN USING LOW PRESSURE POLYETHYLENE AS THE IMPREGNATING POLYMERIC SUBSTANCE. 